This invention relates to a linear antenna array wherein three or more element antennae are arranged along a straight line, and more particularly an antenna array suitable for receiving television waves and frequency modulated signals.
As an television receiving antenna so called unidirectional antenna which is sensitive to electric waves coming from a given direction but not to electric waves in other directions has been used for the purpose of eliminating ghost signals caused by electric waves reflected by buildings. One example of such antenna is disclosed, for example, in Japanese laid open patent specification No. 4146 of 1977 dated Jan. 13, 1977. The most simple unidirectional antenna disclosed therein is a two element dipole array antenna of the phase difference feeding system in which two dipole antenna elements are arranged along a straight line with their antenna conductors disposed in parallel with a spacing d, and two feeder lines l.sub.1 and l.sub.2 having different lengths are connected between respective element antennae and the split ports of a splitting circuit which are divided into two ports from a common feed ports. The phase of either one of these two feeder lines is reversed by, for example, crossing two parallel conductors. When the lengths l.sub.1 and l.sub.2 of the two feeder lines and the spacing d between the antenna elements are selected to satisfy an equation: ##EQU1## where K is a wavelength compression coefficient, the antenna would receive electric waves in the direction of a straight line along which the antenna elements are arrayed, but not electric waves coming from other directions. The unidirectional antenna of the phase difference feeding type is characterized in that the unidirectional performance of the directivity does not vary irrespective of the variation in the frequency of the received waves. In other words, it is possible to obtain a directive antenna over a wide frequency band.
However, in such two element antenna, the direction in which the directivity is the maximum or minimum (null point direction) is only one for each direction. However, the direction of arriving the ghost signals in the reception of television waves is not generally limited to one, and in most cases the ghost signals come from a number of directions. For this reason, in order to eliminate ghost signals, it has been strongly desired to provide an antenna which have a plurality of null directions and which can install the null points in an arbitrary direction. However, until today there was no antenna that can meet such requirement over a wide frequency band.
To satisfy these requirements it is necessary to increase the number of the antenna elements and to use a special method of feeding that assures a desired directivity. According to said Japanese laid open patent specification No. 4146 of 1977, 2.sup.n dipole antenna elements are arrayed along a straight line and the phase difference feed system is used. However, the object of this antenna array is to make more sharper the directivity of the two elements antenna thereby improving the ratio between the maximum and the minimum values of the directivity, that is the front to back ratio characteristic, and this laid open patent specification does not teach the installation of null points in a plurality of difference directions as contemplated by the instant invention.